1. Field of the Invention
The present invention relates to an electromagnetic wave shielding system for an optical sensor having an aperture which is capable of preventing electromagnetic waves from entering and interfering with a signal processing unit.
2. Description of the Related Art
Optical sensors for converting an optical signal into an electrical signal and for electrically processing, e.g., amplifying, the electrical signal are known. Such optical sensors have an optical aperture through which an optical signal passes whereby it is made incident on the sensor. However, the aperture of the optical sensors also allows the passage of electromagnetic waves. In consequence, electromagnetic waves may enter the sensor and interfere with an electrical signal processing unit disposed within the optical sensor.
Optical sensors may be installed in an an artificial satellite in the vicinity of a source of generating electromagnetic waves at a high level. Such optical sensors detect a very weak optical signal, converts the detected optical signal into an electrical signal, and amplifies the electrical signal in a wide range. In that case, a problem involving the interference of electromagnetic waves with the signal processing unit disposed within the sensor often arises.
Conventionally, such an interference of the electromagnetic waves with the signal processing unit is prevented in the manner described below: a conductive film is coated on the surface of the lens which constitutes an optical system for the optical sensor so as to attenuate the electromagnetic waves. Alternatively, a shielding plate is disposed between the optical sensor and the electromagnetic wave generating source so as to prevent the electromagnetic waves from entering the interior of the sensor.
However, in the former case in which a conductive film is coated on the surface of the lens of the optical system, the conductive film must be thickly coated in order to achieve effective attenuation of the electromagnetic waves. However, the thick coating reduces the transmittance of the optical system with respect to an optical signal to be detected, thereby reducing the performance of the optical sensor.
In the latter case where the shielding plate is disposed between the optical sensor and the electromagnetic wave generating source, design of the shielding plate is based on an ad hoc decision, and it is therefore difficult to dispose the shielding plate in such a manner that it does not block the visual field of the optical sensor. Furthermore, a large shielding plate increases the overall weight of the sensor.